To avoid this problem I prefer to leave the bodice side seam open and then stitch the open sleeve to the open armhole. Clipping armhole seam will also help.

You need a gusset! Some shirt patterns include an extra gusset piece, some have added to the underarm portion of the pattern, and I suspect yours has none. But it's an easy pattern alteration. I saw it on Susan Kaljhe's show once.

Its an easy alteration. Set you sleeve pattern before you, with the cap at the top and the cuff at the bottom. Basicaly, you want to extend the ends of the cap outwards, so you are extending the width of the sleeve. Extend maybe 3/4 inch on either side. Then use a straight line to blend these new points into the sleeve lines. Finally, you need to reduce the overall height of you sleeve cap so that the perimeter of the cap is the same as the original. Otherwise you'll just add in more ease.

If you'd like a sketch of this just e-mail me at debrabrun@yahoo.com. It's ingenious and I use it for aal my sleeves - and my shirts don't lift anymore!